Convert a child's hat to adult

I did a search but I can’t find any info on converting a child size hat to an adult size hat. Do I just use bigger needles? Do I cast on more stitches?
When I was knitting in years past a yarn shop owner converrted a baby sweater to my size…It came out neat but after 12 years I still have not put the buttons on it! lol
Thanks for your help!

http://www.mielkesfarm.com/bsic_hat.htm

This site gives you guidelines of how to make a hat with different sizes. Maybe you could use these guidelines.

I would add more stitches and just try to make it proportionately bigger, but this site gives more specific directions.

I hate to do this again, but ladies, this is what we call “knitting math”. You can decide how much bigger you want the finished hat to be. 50% bigger, or 150% bigger. Once you choose a percentage, all the increases and decreases must also change by that percentage.

For example. If you have a pattern that says cast on 100 stitches, and you want to make it 10% larger, you would figure 10% of 100 and add that many stitches to your cast on.

True…“the math” helps…but sometimes it does depend on the pattern…if it’s a k2 p2 rib, or has cables etc, that can make increasing or decreasing the size more complicated.